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Topic: Really Simple LED and Resistor Information for a thickie to understand (Read 362 times)previous topic - next topic

SniffTheGlove

Hi,

Really struggling to understand LEDs and Current limiting resistors and would like some advice. I have ready many pages on various site that discuss this but I am just to confused.

For Example
I have 4 Red LEDs connected in series.

Red LED Details are 2.1Vf @ 20ma

I have used some LED calculators and I am told I need to use a 180 Ohm resistor.
OK, I understand this, but if I do not have a 180 Ohm resistor what would happen if I used ones of a different value. ie 82 Ohms or 330 Ohms

This is what I am trying to understand, if I use an 82 Ohm resistor, what happens to the LEDs? Do they burn out quicker or do they last longer? Do they get brighter or dimmer? Do they get hotter or cooler?

The same for using a 330 Ohm resistor?

Also, is a 82 Ohm resistor a higher value than 330 Ohms or a lower value? I am not sure on the scale used for resistors as some people use 1 to 5 with 5 being the higher value and some people use 1 -5 with 1 being the higher value.

Thank you

larryd

Smaller value resistor = more current, more light, more heat, shorter life.
This is relative though.
82 < 330
Plug the resistor values into the formula to see the resulting current flow.

82, 100, 150, 180, 220, 270, 330 some values.
No technical PMs.
If you need clarification, ask for help.

pmckeon

To expand a little on LarryD's answer.

In general you want to avoid exceeding the IF Max (max current) of the LEDs, so going for a higher resistance (keyword there, they are resisting current, so more resistance = less current) is preferable.

If you use your 330 ohm resistor you will have roughly half the current (10.91ma) so half the brightness.

What you could potentially do is use two 330 ohm resistors in parallel, this will actually halve the total resistance to 165 ohms giving you 21.82ma through the LEDs, slightly exceeding their max rating not sure if enough to kill them though.

Another option would be the add a 5th LED in series which requires exactly 82 ohms resistor value.

Hope that helps.

Grumpy_Mike

Quote
If you use your 330 ohm resistor you will have roughly half the current (10.91ma) so half the brightness.
No, the brightness of an LED is not directly proportional to the current. You half the current and the les is only about three quarters bright. The eye has a logarithm response.

SniffTheGlove

I thank you so much for your replies.

So much of what is on the internet makes the assumptions that you already know what is being discussed, even some tutorials for beginners also make this assumption, so you all helping me is wonderful.

Thanks, this will now help me make some illumination for my action cams

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